The invention relates to a drying cylinder for a paper-making machine. Such a machine is shown in U.S. Pat. No. 3,724,094.
A known drying cylinder comprises a cylinder jacket, two end covers closing the jacket, a respective bearing journal at each end cover at which the cylinder is rotated, and a hollow shaft inside the cylinder which links the covers and the journals. Drying cylinders of this type are particularly useful in large diameters, e.g. as creping cylinders and machine glazing cylinders. The hollow shaft increases the strength of the cylinder and prevents the cover from moving apart as a result of the internal steam pressure.
It is known to introduce steam into the cylinder through the journals. In this case, steam first flows through the hollow shaft and then exits through openings in the hollow shaft into the region of the cylinder walls in the outer annular chamber around the shaft, where the steam condenses as a result of the emission of heat. The resultant condensate is conveyed via siphons (suction nozzles) back to the cylinder axis and is discharged from the cylinder by axial pipes arranged in the journal.
The transfer of heat from the steam chamber to the walls of the cylinder improves and becomes more uniform as the suction nozzles (siphons) are more uniformly distributed over the inner surface of the cylinder jacket. This transfer of heat is best if a very large number of small suction nozzles, each having an internal diameter of approximately 4 to 5 mm, for example, are distributed as uniformly as possible over the inner surface of the cylinder. However, these small suction nozzles are sensitive to obstruction. Solid materials, which have been introduced via the steam line, can build up slowly in the nozzles and produce obstructions. When individual nozzles are obstructed, the uniformity of the heat transfer is adversely affected.
Also, as the steam enters the cylinder, it may already be partially condensed, in whch case the condensate entering the cylinder can cause the heat transfer to be impaired and rendered non-uniform.